The present invention generally relates to medical devices for implantation in a human or animal body which are provided with improved tissue and blood biocompatibility. More specifically, metal or glass portions of the medical device are provided with a surface which has been chemically modified with covalently attached bioactive molecules.
Medical devices which serve as substitute blood vessels, synthetic and intraocular lenses, electrodes, catheters and the like in and on the body or as extracorporeal devices intended to be connected to the body to assist in surgery or dialysis are well known. However, the use of such biomaterials in medical devices can stimulate adverse body responses, including rapid thrombogenic action. Various plasma proteins play a role in initiating platelet and fibrin deposition on plastic surfaces. These actions lead to vascular constriction to hinder blood flow, and the inflammatory reaction that follows can lead to the loss of function of the medical device.
A "biomaterial" may be defined as a material that is substantially insoluble in body fluids and that is designed and constructed to be placed in or onto the body or to contact fluid of the body. Ideally, a biomaterial will not induce undesirable reactions in the body such as blood clotting, tissue death, tumor formation, allergic reaction, foreign body reaction (rejection) or inflammatory reaction; will have the physical properties such as strength, elasticity, permeability and flexibility required to function for the intended purpose; can be purified, fabricated and sterilized easily; will substantially maintain its physical properties and function during the time that it remains implanted in or in contact with the body.
As used herein, the solid surface of a biomaterial is characterized as "biocompatible" if it is capable of functioning or existing in contact with biological fluid and/or tissue of a living organism with a net beneficial effect on the living organism. Long term biocompatibility is desired for the purpose of reducing disturbance of the host organism. One approach to improved biocompatibility for biomaterials is to attach various "biomolecules" which can promote the attachment and growth of a normal cell or protein layer such that the body accepts the device as a normal part of the body. Biomolecules such as growth factors and cell attachment proteins which have been attached to the device surface could be used for this purpose. In addition, biomolecules such as antithrombogenics, antiplatelets, anti-inflammatories and the like have also been used to improve the biocompatibility of surfaces.
A number of approaches have been suggested to attach such biomolecules. One such approach is set forth in Dekker et al., "Adhesion of endothelial cells and adsorption of serum proteins on gas plasma-treated polytetrafluoroethylene", Biomaterials, vol. 12 Mar. 1991. In that approach, PTFE substrates were modified by radio frequency plasma to improve the wettability of the surface. Human serum albumin, human fibronectin, human immunoglobulin and human high-density lipoprotein were adsorbed to the plasma-treated substrates followed by seeding with human endothelial cells. Another approach is described in U.S. Pat. No. 5,055,316 to Hoffman et al in which serum proteins such as albumin, immunoglobulins, fibrinogen or fibronectin, or proteins from different sources such as protein-A or glycoproteins are bound to a surface by first using plasma gas discharge in the presence of a plasma-polymerizable fluorinated hydrocarbon gas to provide a plasma-deposited surface, followed by exposure to a solution of the protein. Covalent attachment of such biomolecules can be found in Ito et al., "Materials for Enhancing Cell Adhesion by Immobilization of Cell-Adhesive Peptide", Journal of Biomedical Materials Research, 25:1325-1337 (1991) in which fibronectin or RGD peptide are bonded to the hydrogel by the use of a water soluble carbodiimide. Although this method allows coupling of the biomolecule extended from the surface, the fact that the biomolecule is immobilized throughout the gel layer reduces the availability of the biomolecule for interaction with, for example, cells intended to adhere to the biomolecule.
Spacer molecules have been used to address this problem. A spacer molecule is a molecule or compound which is capable of attachment to a solid surface, is large enough to extend from the surface of said surface and is capable of immobilizing a biomolecule and/or biomolecules. The spacer insures that the active site of the biomolecule is held outward away from the support so as to contact the body fluid efficiently. The spacers are derived from organic molecules having at least two reactive functional groups, or more, generally situated at opposing ends of the molecule. Such groups serve as attachment vehicles capable of coupling the spacer to the solid surface and to the biomolecule. For example, in U.S. Pat. No. 5,132,108 to Narayanan et al., a copolymer surface was subjected to radio frequency plasma treatment by subjecting it to a radio frequency electric field in the presence of a water vapor plasma medium. An aqueous solution of polyethyleneimine (PEI) and 1-(3-dimethylpropyl)-3-carbodiimide (EDC) coupling agent was applied to the radio frequency plasma discharge modified polyurethane surface. An aqueous solution of heparin and EDC was then applied to the PEI-treated surface in order to provide a polymeric surface having an anti-thrombogenic agent secured to its surface. However, considering the heterogeneity of the polyurethane surface even coating with the multi-functional spacer molecule is not guaranteed.
Additional coverage can be provided, for example, according to U.S. Pat. No. 4,565,740 to Colander et al. or U.S. Pat. No. 5,049,403 to Larm et al. In the first of these patents, a complex of a polymeric cationic surfactant (e.g. a polyalkyleneimine) and a dialdehyde (e.g. glutaraldehyde) is adsorbed onto a substrate material. In the second of these patents, a polyamine is adsorbed onto the surface of a substrate and crosslinked with crotonaldehyde. Multiple coatings, including intermediate layers of anionic material are then applied to obtain an effective coating. However, these crosslinked coatings rely on adsorption onto the surface and ionic bonding to the surface, which may not provide good bonding of the coating to the surface.
The inventors of the present invention have contributed to improvements in biocompatibility of biomaterials through the use of multilayer coatings in their U.S. Pat. Nos. 5,229,172; 5,308,641 and 5,350,800. For example, in U.S. Pat. No. 5,229,172, we discovered a method for modifying the surface characteristics of a polymeric material by providing a base layer of grafted acrylamide on the polymeric surface which can be used to attach various spacers and biomolecules. Or, in U.S. Pat. No. 5,308,641, we discovered an improved spacer material which includes a polyalkyeneimine covalently attached to an aminated substrate and crosslinked with a crosslinking agent which is difunctional in aldehyde groups. Or, in U.S. Pat. No. 5,350,800, we discovered a method for attaching a biomolecule having carboxyl groups to an aminated solid surface by a carbodiimide and then selectively restoring the bio-functionality of the carboxyl groups.
On metal or glass surfaces, the binding of the base layer of such multi-layer coatings can be a problem since there is no organic structure to provide covalent bonds between the metal or glass substrate and the grafted base layer. Others have addressed the problem of binding to metals and glass by applying aminosilanes to adhere to the surface and then attaching the biomolecule to the aminosilane through the amine functionality of the aminosilane. This can be seen in U.S. Pat. No. 5,355,433 issued to Rowland et al in which an aminosilane is used to adhere a heparin molecule to an oxidized tantalum surface. Aminosilanes are also disclosed for attachment of a heparin molecule to glass or metal surfaces in U.S. Pat. No. 4,118,485 issued to Eriksson et al. However, the use of aminosilanes in coatings of this sort has not been very good in producing a surface with a high level of both bioeffectiveness and stability.
It is therefore an object of the invention to provide a base for the attachment of biomolecules and/or spacer molecules with improved stability on metal or glass substrates.
It is also an object of the invention to provide a combined base/spacer which presents a stable platform for the attachment of the biomolecule and thereby prevents the attached biomolecule from being buried in the spacer layer.